1. Field of the Invention
The present invention relates to transistor biasing, and in particular to transistor biasing independent of inductor voltage drop.
2. Related Art
Transistors are frequently used in radio frequency (RF) applications. Transistor biasing can advantageously facilitate amplification. A conventional amplifier can include various components to control its gain and distortion. For example, inductors can be used to provide a high impedance to alternating current, serving to “choke” off RF from DC supply lines in the amplifier. Such a high impedance inductive load may also serve to provide common mode rejection when used in the common node of a differential amplifier. Such an RF choke can be designed to have a high impedance over a wide range of frequencies. Unfortunately, biasing transistors with inductors in their source/emitter legs and using a replica current mirror circuit (a known implementation) can result in inaccurate biasing because of a voltage drop across one or more inductors.
One approach to solve this voltage drop problem is to create an equal voltage drop in the replica current mirror circuit using a metal resistor. This metal resistor must be large enough to match the inductor (or even larger if the replica current mirror circuit is a scaled, i.e. lower current, version for power saving). Unfortunately, this metal resistor may take up significant area on the integrated circuit.
Further, differential amplifiers have commonly used a constant voltage reference connected through resistors to produce a fixed voltage on the common current point which, when attached to a resistor to, for example, a ground potential, will produce a constant current equal to the sum of the currents in the output legs of the amplifier. This voltage drop across the resistor must necessarily be less than the total voltage drop across the differential amplifier, thereby limiting the amplifier's output voltage swing. Ever-decreasing feature sizes dictate ever-decreasing supply voltages, thus limiting the output voltage swing that may be achieved using existing biasing designs.
Therefore, a need arises for an amplifier that can accurately set its bias current independent of a voltage drop across an inductor. There is a further need for an amplifier circuit having a constant current design which maximizes the available voltage swing at its output for a given supply voltage.